This invention relates to an optical fiber amplifier wherein signal light and pumping light are introduced into a rare-earth element doped fiber doped with a rare-earth element to directly amplify the signal light.
In an optical fiber communication system which is in practical use at present, repeaters are interposed at predetermined intervals in order to compensate for attenuation of a light signal which may be caused from a loss of an optical fiber. At each repeater, a light signal is converted into an electric signal by means of a photodiode and then the electric signal is amplified by an electronic amplifier and then converted again into a light signal by means of a semiconductor laser or the like, whereafter the light signal thus obtained is forwarded again into the optical fiber transmission line. If such light signal can be directly amplified as it is with a low noise, then the optical repeaters can be reduced in size and economized.
Thus, researches for optical amplifiers which can directly amplify a light signal are being proceeded extensively, and optical amplifiers which are objects of such researches are roughly classified into three types including (a) optical amplifiers wherein pumping light is combined with an optical fiber which is doped with a rare-earth element (Er, Nb, Yb or the like), (b) optical amplifiers which include a semiconductor laser doped with a rare-earth element, and (c) induction Raman amplifiers and induction Brillouin amplifiers which make use of a non-linear effect in an optical fiber.
Among those optical amplifiers, the first optical amplifiers of (a) wherein pumping light is combined with a rare-earth element doped fiber (hereinafter referred to simply as doped fiber) have superior features that they have no polarization dependency, that noises are low that the coupling loss of a transmission line is low. Thus, it is expected that the transmission repeater distance in an optical fiber transmission system can be increased remarkably and a light signal can be distributed to a large number of stations.
A principle of optical amplification in a doped fiber is illustrated in FIG. 1. Referring to FIG. 1, an optical fiber 2 is constituted from a core 4 and a clad 6, and erbium (Er) is doped in the core 4. If pumping light is introduced into such erbium doped fiber 2, then erbium atoms are excited to a high energy level. If signal light comes at the erbium atoms in the optical fiber 2 excited to a high energy level in this manner, the erbium atoms are changed into a ground state, whereupon stimulated emission of radiation of light occurs. Consequently, the power of the signal light is increased gradually along the optical fiber so that amplification of the signal light is effected.
General construction of an exemplary one of conventional optical fiber amplifiers which employ such principle is illustrated in FIG. 2. Referring to FIG. 2, the optical fiber amplifier is generally denoted at 8 and includes an optical fiber 10 doped with erbium, an incidence side optical fiber 12, and an emergence side optical fiber 14. The incidence side optical fiber 12 and the doped fiber 10 are optically coupled to each other using a pair of lenses 16 while the doped fiber 10 and the emergence side optical fiber 14 are also optically coupled to each other using another pair of lenses 16.
The optical fiber amplifier 8 further includes a pumping light source 18 such as a laser diode which emits pumping light of a wavelength of, for example, 1.48 .mu.m. Pumping light emitted from the pumping light source 18 is coupled to signal light from the incidence side optical fiber 12 by means of an optical coupler 22 by way of a lens 20. When the optical power of the pumping light is sufficiently high, erbium atoms in the doped fiber 10 are put into an excited state, and if signal light is introduced to the erbium atoms, then light is stimulated to emit from the erbium atoms. Consequently, the optical power of the signal light is increased gradually along the doped fiber 10, that is, the signal light is amplified, and the thus amplified signal light is introduced into the emergence side optical fiber 14.
In order to achieve sufficient optical amplification, the pumping light source 18 is required to have a high power of such as, for example, several hundreds mW. However, in case it is difficult to obtain such high power with the single pumping light source 18, a pair of pumping light sources 18 and 24 may be used as shown in FIG. 3. In particular, pumping light from the pumping light source 24 is optically coupled to pumping light from the pumping light source 18 by means of an optical coupler 28 by way of lens 26. The pumping light coupled in this manner is then coupled to signal light from the incidence side optical fiber 12 by means of another optical coupler 22 and introduced into the doped optical fiber 10.
Where optical amplification is effected with an optical fiber doped with a rare-earth element such as erbium, conventionally a pumping light source of a high power of, for example, several hundreds mW or so is required as described above, and in order to make up for an insufficient power, normally a plurality of pumping light sources are used to introduce pumping light therefrom into the doped fiber. Consequently, in order to assure a high power of a pumping light source such as a laser diode, high driving current is required, which sometimes deteriorates the reliability of a device. Further, since a plurality of pumping light sources are used, an introducing mechanism for pumping light is complicated, which deteriorates the economy of the system. In addition, in case the output power of a pumping light source is low, the length of a doped fiber must be increased, for example, to several tens meters or so, and accordingly, there is a problem that miniaturization of the optical amplifier cannot be achieved.